OSI was the first conceptual model that characterized and standardized the communication functions of a Network system without regard to its underlying internal structure and technology.
OSI is a Vendor Neutral Model. Before OSI Model, each vendor used to use its own network model to manufacture Network equipment. So, one vendor’s equipment & network was not compatible with another vendor’s equipment. Buyers were bound to buy only from one vendor.
For example, if you were buying IBM Computers then you were now bound to buy all Network Equipment from IBM as well in the future. Because IBM Computers couldn’t be connected by other vendor Switches or Routers. You must buy IBM Switches to connect them. They will not work with HP Server or Cisco Switches.
To resolve this problem, the idea of a standardised Network Model emerged. International Organization for Standardization (ISO) started working on it, different network models were proposed and finally the OSI Model was chosen & released in 1984 by ISO. OSI provided brought in the flexibility along with standardization.
In simple words, OSI reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer.
Layers in OSI Model
OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it … which we will discuss in detail later. OSI Model has 7x Layers starting from Physical till Application level as in below:
Physical Layer standardises the Physical medium. For example, signal level, voltage, radio, cabling standards & so on…. It simply, standardises everything which you can touch & feel. Physical layer is responsible for ultimate transmission of digital data bits in the form of electrical, optical or radio signals from the Physical layer of the source or sending device over network communications media to the Physical layer of the receiving of destination device. Data Link Layer standardizes the Physical Addressing. It is responsible for Data Switching. Physical addressing includes MAC Address & Ethernet Frames. Network Layer deals with Logical Addressing. It is responsible for Data Routing. Transport Layer ensures the error free data transmission across networks. It mainly focuses on data delivery. Transport Layer mainly uses two protocols called TCP & UDP which we will discuss in detail in later sections. Session Layer is responsible for session control. In simple words, it manages on how to start, control, and end conversations between two hosts or computers. Presentation Layer is responsible for data organization & conversion so that it is readable by Application layer of all hosts/applications involved in the data communication. Application Layer is responsible for end-user processes or applications. It supplies network services to end-user applications.